To accurately grasp the charge state of a secondary battery, it is important to accurately grasp the open-circuit potential of an electrode of the secondary battery. It is known of the secondary battery that the open-circuit potential of an electrode changes in accordance with the initial charge amount of the electrode at the time when charging or discharging starts. That is, in a calculation apparatus, to accurately grasp the open-circuit potential of an electrode of the secondary battery, a plurality of data must be stored in advance for each of various initial charge amounts. A large storage area must be ensured.